A Pesticide Information Project of Cooperative Extension Offices of
Cornell University, Michigan State University, Oregon State University, and
University of California at Davis. Major support and funding was provided
by the USDA/Extension Service/National Agricultural Pesticide Impact
Assessment Program.

TRADE OR OTHER NAMES

REGULATORY STATUS

Products containing pendimethalin must bear the signal word "Caution" or
"Warning" depending on the formulation.

INTRODUCTION

Pendimethalin is a selective herbicide used to control most annual
grasses and certain broadleaf weeds in field corn, potatoes, rice, cotton,
soybeans, tobacco, peanuts and sunflowers. It is used both preemergence, that
is before weed seeds have sprouted, and early postemergence. Incorporated
into the soil by cultivation or irrigation is recommended within 7 days
following application. Pendimethalin is available as emulsifiable
concentrate, wettable powder or dispersible granule formulations (1, 6).

TOXICOLOGICAL EFFECTS

ACUTE TOXICITY

Pendimethalin is slightly toxic if ingested, inhaled or absorbed through
the skin. The most probable occasion for human exposure is to applicators
during mixing, loading, spraying and flagging (5). Pendimethalin is a mild
skin irritant (6). When applied to the eyes of rabbits, pendimethalin caused
irritation of the cornea which cleared within 7 days. Inhalation of dusts or
fumes may be mildly to moderately irritating to the linings of the mouth,
nose, throat and lungs (2).

The amount of a chemical that is lethal to one-half (50%) of experimental
animals fed the material is referred to as its acute oral lethal dose fifty,
or LD50. The oral LD50 for technical pendimethalin in rats is greater than
5000 mg/kg (6). The dermal LD50 for technical pendimethalin in rabbits is
greater than 2000 mg/kg.

The lethal concentration fifty, or LC50, is that concentration of a
chemical in air or water that kills half of the experimental animals exposed
to it for a set time period. The inhalation 4-hour LC50 for technical
pendimethalin in rats is 320 mg/l and for Prowl 4EC is 3.5 mg/l (6, 2).

CHRONIC TOXICITY

Increases in alkaline phosphatase level and liver weight were produced in
dogs fed 50 and 200 mg/kg for 2 years. No effects were observed at 12.5
mg/kg/day (2, 4). In a 90 day feeding study of rats, the NOEL was 500 ppm (40
mg/kg/day) (4).

Reproductive Effects

Slightly fewer offspring with decreased weight gain from weaning to
maturity were observed in a 3-generation reproductive study of rats tested at
levels up to 250 mg/kg. No effects were observed at 30 mg/kg (40 mg/kg) (2, 4).

Teratogenic Effects

No birth defects and no toxic effects on fetuses occurred when pregnant
rats were given 500 mg/kg, the highest dose tested. The NOEL and the highest
dose tested in a teratology study with rabbits was 60 mg/kg (4).

Mutagenic Effects

EPA reports that several mutagenicity studies, including tests on live
animals and mammalian and bacterial cell cultures, have all shown that
pendimethalin is not mutagenic (4).

Carcinogenic Effects

EPA is currently reviewing the carcinogenicity data for pendimethalin.

Organ Toxicity

Increases in alkaline phosphatase level and liver weight were produced in
dogs fed 50 and 200 mg/kg for 2 years (2, 4).

Fate in Humans and Animals

By 24 hours after the administration of 37 mg/kg of radio-labeled
pendimethalin to rats, 90.3% of the dose was recovered in the feces and urine.
After 96 hours, 95.8% of the dose was recovered in the urine (20.9%) and feces
(74.9%). When a lower dose was administered (7.3 mg/kg), 99.8% was recovered
in the urine (21.8%) and feces (78.0%) after 12 hours. After 96 hours,
residues were less than 0.3 ppm in all body tissues except fat, which had 0.9
ppm. This study indicates that ingested pendimethalin is largely unabsorbed
by the bloodstream and excreted through the feces. Pendimethalin which does
become absorbed into the bloodstream from the gastrointestinal tract is
rapidly metabolized in the kidneys and liver and is then excreted in the urine
(7).

ECOLOGICAL EFFECTS

Effects on Birds

When used according to label instructions, pendimethalin is not toxic to
birds (6). The 8-day dietary LD50 for pendimethalin in bobwhite quail is
greater than 3,149 ppm, and for mallard ducks is greater than 4,640 ppm (5).

Effects on Aquatic Organisms

Pendimethalin is highly toxic to fish and aquatic invertebrates. Runoff
from treated areas may be hazardous to aquatic organisms in neighboring bodies
of water. Pendimethalin should be kept out of lakes, streams and ponds. Do
not contaminate open waters during cleaning of equipment or disposal of wastes
(1, 5). The 96-hour LC50 for pendimethalin in bluegill sunfish is 0.199 ppm,
and for rainbow trout is 0.138 ppm. The 48-hour LC50 in Daphnia magna, a
small freshwater crustacean, is 0.28 (5).

Effects on Other Animals (Nontarget species)

When used according to label instructions, pendimethalin is not toxic to
bees or mammals (6).

ENVIRONMENTAL FATE

Breakdown of Chemical in Soil and Groundwater

Pendimethalin adsorbs strongly to soil organic matter and clay and does
not leach through the soil to contaminate groundwater (3, 6, 5).

Pendimethalin is not subject to microbial degradation. Slight losses of
pendimethalin can result from photodecomposition and volatilization. When
used according to label instructions, no carry over to subsequent crops is
expected (6). Its soil half-life is 90 days (3).

Breakdown of Chemical in Water

Pendimethalin is stable to hydrolysis, but is degraded by light in
aquatic systems. Pendimethalin may be removed from water by its strong
tendency to bind to sediment and organic matter. It is rapidly degraded in
sediment.

Breakdown of Chemical in Vegetation

Pendimethalin's herbicidal effects are related to inhibition of cell
division and cell elongation. It is absorbed by plant roots and shoots.
Pendimethalin is not absorbed by the leaves of grasses. Only very small
amounts are taken up by plants from the soil. Once absorbed into plant
tissues, translocation is limited and pendimethalin breaks down via oxidation.
Residues on crops at harvest are usually below detectable levels (0.05 ppm)
(6).

PHYSICAL PROPERTIES AND GUIDELINES

Pendimethalin is an orange-yellow crystalline solid with a faint nutty or
fruit-like odor (3, 6). Products containing pendimethalin range from slightly
to highly flammable. They should not be stored, used or handled near heat or
open flames (6). It is slowly decomposed by light (2). It is stable under
normal temperatures and pressures, but it may pose a slight fire hazard if
exposed to heat or flame. It poses a fire and explosion hazard in the
presence of strong oxidizers. Thermal decomposition of pendimethalin will
release toxic oxides of nitrogen and carbon (2).

Occupational Exposure Limits:

No occupational exposure limits have been established for pendimethalin
by OSHA, NIOSH or ACGIH (3).